Animation - Using a Different Axis to Rotate the Entire Object Than the Normal Object Axis
One simple way to take care of this is to rotate by the 3D cursor. If you know the exact global space coordinates where you would like this point of rotation to be, you can enter them in the View tab from the Properties Shelf, to the right of the 3D View (press n to bring up the Shelf if not exposed currently), and under the 3D Cursor section, there are a set of transform fields. By entering the desired transform values in these fields, the 3D cursor will move to the appropriate worldspace coordinates. Alternatively, you can snap the 3D cursor to another object, provided you have a selected object whose origin is currently where you want the 3D cursor to be. You would do this by accessing the Snap menu (Shift + s) and choosing Snap Cursor to Selected. Once you have the 3D Cursor where you want the new point of rotation to happen from, press the Transform Pivot Point button at the top of the 3D View, and from the pulldown, choose 3D Cursor. Now, when you rotate the selected object (or indeed any other thing), it will use the 3D Cursor's current position as the point of rotation.